This invention pertains to the art of safety lock devices and, more particularly, to safety lock devices of the type for automatically locking an inner elongated member against movement toward a fully telescoped position within an outer member. The invention is particularly applicable for use on vehicle lifts or the like and will be described with reference thereto. However, it will be appreciated that the invention has broader aspects and the safety lock device may be used on other apparatus having longitudinally movable inner and outer members, such as telescoping ladders or cranes, or the like.
Vehicle lifts or the like commonly include a supporting platform which is raisable and lowerable by operation of a fluid cylinder. The supporting platform is selectively elevated to any desirable elevated position by operation of the fluid cylinder. The elevation to which the platform is raised depends upon the work to be done on the vehicle supported by the platform, and upon the personal desires of the person doing the work.
In devices of the type described, a safety lock is provided for preventing lowering movement of the platform from its elevated position in the event of a failure in the fluid cylinder, or in the event the lowering control for the fluid cylinder is accidentally operated while a workman is beneath the platform. One old type of safety device simply comprises a bar pivoted to the platform for movement to a vertical position for preventing lowering movement of the platform from its fully elevated position. A safety device of this type is inoperative when the platform is raised to elevations less than the highest elevation.
In order to prevent lowering movement of a platform from any of a plurality of elevated positions, many different types of safety lock devices have been proposed. For example, U.S. Pat. No. 1,938,191 issued Dec. 5, 1933, to Metz, et al. discloses a safety lock device which is cooperable with the elevating fluid cylinder. A cam ring surrounding the inner member of the fluid cylinder receives a plurality of balls which are cammed into firm engagement with the inner member of the fluid cylinder for preventing lowering movement thereof unless the cage carrying the balls has been manually elevated to prevent camming engagement of the balls with the inner member. Balls essentially make point contact with the inner member and place high stress concentrations thereon. In addition, balls have a tendency to easily slip upwardly on the cam ring or to rotate so the inner member may not be firmly locked against lowering movement. The safety lock device in Metz is exposed externally so it is subject to damage, and can accumulate significant amounts of dirt and grease. It would be more desirable to have a safety lock device carried by the inner member internally of an outer member for somewhat protecting the safety lock device from damage, and from accumulating significant quantities of dirt and grease. The Metz lock device is manually released by pulling on a cable for rotating the ball cage which then cooperates with cams on the cam ring for vertically lifting the ball cage. The cable must be constantly tensioned while the lift is lowered so there is no stable releasing position. Wrapping the cable around a fixed member for lowering the lift could result in a safety hazard when the lift is again elevated if a workman forgets to release the cable. It would be desirable to have a safety lock device which is automatically armed upon movement of the lift from any elevated position to a retracted position so that it will be automatically operated upon the next elevating movement on the lift.
Other prior patents of interest include U.S. Pat. No. 2,608,381 issued Aug. 26, 1952, to Pelouch who discloses a lever which is spring biased to a locking position. However, movement of the lever to its locking position requires manual movement of a pivoted locking member and the lever is only for locking the lift in its fully elevated position.
U.S. Pat. No. 2,684,129 issued July 20, 1954, to Jaseph discloses a locking device having a spring biased pivoted lever for moving locking members into and out of their locking positions. The Jaseph device also locks only in the fully elevated position of the lift.
U.S. Pat. No. 2,849,084 issued Aug. 26, 1958 to Hott, et al. discloses a safety lock device including an elongated ratchet cooperating with a pivoted pawl for locking the lift in any elevated position. Arrangements of this type are very expensive due to the necessity of making a toothed ratchet which must have hardened teeth. The impact load on the pawl and teeth is extremely high in the event of a failure in the elevating mechanism.
U.S. Pat. No. 2,954,100 issued Sept. 27, 1960, to Harr discloses a safety lock device including toothed tubular member cooperable with a pivoted pawl. In the Harr device, the pivoted pawl and its mechanism are externally exposed so they are subject to damage, along with accumulation of dirt and grease which may prevent reliable operation. In addition, the teeth on the tubular member are relatively widely spaced so a great impact load would be imposed upon the teeth and the pawl in the event of a failure in the elevating mechanism. Manufacture of a toothed tubular member is also very expensive.
U.S. Pat. No. 2,956,644 issued Oct. 18, 1960, to Greene discloses a safety lock device which is engaged only at the uppermost elevated position of the lift. The locking device is spring biased to a locking position upon elevation of the lift but engages only upon substantially full elevating movement of the lift.
Other patents of interest for locking devices which are engaged upon elevation of a vehicle lift include: Racely--U.S. Pat. No. 3,013,635 issued Dec. 19, 1961; Jaseph--U.S. Pat. No. 3,056,468 issued Oct. 2, 1962; Wallace, et al.--U.S. Pat. No. 3,098,542 issued July 23, 1963; and, Quatkemeyer--U.S. Pat. No. 3,363,724 issued Jan. 16, 1968. In these patents, a locking lever is moved to a locking position upon movement of the lift to its elevated position. However, the locking devices are not capable of locking the lift in any elevated position in the event of a failure in the lifting mechanism. Vehicle lifts of the type described may be operated in such a manner that they are elevated above a supporting surface between positions wherein a mechanic can work on a vehicle while lying on his back, and a position wherein the mechanic can work on the vehicle while standing.